Nowadays, with the booming economy, most countries in the world are facing a common problem of freshwater shortage. Although two thirds of the earth surface is covered by water, 97% of the water storage is seawater and bitter salty water. Among the rest 2.53% of freshwater, 77% is distributed on the two poles of the earth and mountain glaciers, so the amount of freshwater directly available for human use is less than 0.7% of the water storage on the earth.
Water in substance form is rich on the earth, such as seawater. If the seawater can be converted into freshwater through separation, the problem of water shortage can be completely solved. Recently, more than 120 countries and areas in the world acquire freshwater by desalting seawater or bitter salty water.
Currently, seawater desalination techniques adopted worldwide are thermal and membrane methods, also known as distillation and reverse-osmosis processes. The corresponding liquid separating devices are distillation separating device and reverse-osmosis separating device respectively.
1. Distillation Separating Device:
i. Multi-stage flash (MSF) device: with an operating temperature, a gain output ratio (GOR), and a stage of 120° C., 10, and 40 respectively. In addition to a certain amount of heating steam, the MSF device also consumes an electric power of 4 KWh to 5 KWh per 1 m3 of freshwater for seawater circulation and liquid transportation, so the power consumption is large.
ii. Low temperature multi-effect (LT-MDE) device: performing distillation on the basis of multi-effects, with a temperature lower than 80° C., the number of effects around 12, and a GOR higher than 10. In addition to a certain amount of heating steam, the LT-MDE device also consumes an electric power of 1.8 KWh/m3 for liquid transportation, so the top two factors affecting the cost are the steam and then the power consumption, and the cost is high.
2. Reverse-Osmosis Separating Device:
The reverse-osmosis separating device is designed to force the seawater into a reverse-osmosis membrane by a certain pressure (60 kg/cm2). The membrane of this device only allows water molecules in the seawater to pass, and holds back most of the salt molecules, thus obtaining the freshwater.
The reverse-osmosis separating device consumes an electric power of 3 KWh to 4 KWh per 1 m3 of freshwater, and the top two factors affecting the cost are the power consumption and expenses for replacing the medicament and membrane, so the manufacturing cost and use fee are high.
In view of the above, the separating devices in the aforementioned methods all have a complicated structure, large space occupation, high power consumption, and great electric power charge. Therefore, those defects should be eliminated.